Cpu vs gpu vs ram rendering in blender
Well, everything depends on your scene. 

An 8GB graphics cards will be faster than than your CPU but you will be limited to 8GB (side note: do not keep in your old 2GB graphics card, along with the new 8GB card, Blender will only use the ram of the lowest card) 

If you go with the CPU 16GB ram then you will be able to render larger scenes but it will be slower. 

I would say get the graphics card with 8GB.